The invention is directed to a method of thermally removing binder from porous compacts pressed from metallic or ceramic materials using atmospheric pressure. The invention is further directed to binder-free compacts produced with the method.
Porous, sintered powder metallurgy and ceramic compacts have been used for many years for a wide variety of applications, including fluid filters (for both liquids and gases), oil mist eliminators, catalyst beds, electrolytic lightning arresters, electrodes for gas lighting and strobe light tubes, for oil-impregnated bearing surfaces, and for electrolytic capacitor anodes.
While some metal powders and a few ceramic powders can be pressed to significantly less than the theoretical densities of the materials without the use of a binder to help hold the pressed compacts together, most materials cannot be so-compacted successfully. Because of the poor strength of powder metallurgy or ceramic material compacts pressed at sufficiently low densities to yield significant pressed-compact porosity, most compacts pressed from these materials contain a binder which aids in increasing the pressed-compact strength prior to a sintering operation. Binders also act to lubricate the punches and dies during the pressing operation, thereby extending tooling life and minimizing downtime for press repairs/tooling changes. The presence of a binder within the pores of a powder metallurgy or ceramic compact also helps to provide an open pore structure at a given as-pressed density, resulting in effectively increased compact porosity.
For many applications, such as filters, bearings, oil mist eliminators, and gas tube electrodes, a small amount of binder decomposition products retained within the bodies of the sintered compacts presents no problem with respect to the end use of the sintered compacts. For other applications, such as catalyst beds or electrolytic capacitor anodes, binder residues, usually present as carbon or metallic carbides, present significant problems with respect to the end use of the sintered compacts. The residual carbon contamination resulting from binder decomposition/reaction tends to give rise to flaws in the anodic oxide films grown on sintered valve metal compacts used as electrolytic capacitor anodes, for example, leading to increased leakage current and short-circuit failures of the finished devices containing flawed dielectric oxide films. Although there appears not to be a threshold value for carbon contamination for tantalum anodes, for example (i.e., there does not seem to be a carbon content from binder residuals below which there is no problem and above which the onset of problems is observed), it is generally agreed upon by those in the industry that carbon should be below 100 ppm in sintered tantalum anodes and preferably as low as possible.
The highly variable purity requirements for sintered compacts, depending on the end use of the parts, has led to the use of a wide variety of binder materials. Powder metallurgy or ceramic powder compacts for those applications not requiring a low residual post-sintering carbon content are frequently pressed with binders such as paraffin, polyethylene Glycol 8000, glyptal-brand glycerine polyester, etc. For those applications requiring low residual post-sintering carbon levels, but where the particle size of the material is relatively coarse (e.g., 10 microns and larger) and the material is of a relatively inert nature (bronze, stainless steel, etc.), the above binders are still found to have merit.
For applications involving reactive materials and having low post-sintering residual carbon requirements such as tantalum powder metallurgy electrolytic capacitor anodes, there has been an ongoing search for binders having lubricity during the pressing operation and sufficiently high vapor pressure for ready removal at elevated temperatures.
One binder material used for many years in the fabrication of tantalum electrolytic capacitor anodes is ethylene diamine bis-disteramide, sold under the brand name xe2x80x9cAcrawaxxe2x80x9d (manufactured by the Lonza Corp.) With tantalum powders having low to medium surface area, e.g., 25,000 microfarad volts/gram or approx. 0.25 square meters/gram (sintered surface area), vacuum distillation at temperatures of up to 400xc2x0 C. or more will reduce the residual binder content to the point that a post-sintering anode carbon content below 100 ppm is readily achievable.
As the demand for higher surface area tantalum powders (for purposes of economy and volumetric efficiency) has led to the introduction of powders having surface areas (in the sintered state) in excess of 0.5 square meter per gram and CV products in excess of 50,000 micro farad volts (micro coulombs) per gram, it has become increasingly more difficult to reduce the residual post sintering carbon content to acceptable levels. Traditional binders, such as Acrawax , stearic acid, camphor, etc., partially decompose during vacuum distillation from high surface area tantalum powders, resulting in post-sintering carbon levels in excess of 100 ppm.
In the production environment found in the powder metallurgy electrolytic capacitor industry, it has proven impractical to press high surface area tantalum and other valve metals and valve metal compounds (such as nitrides and suboxides, etc.) into capacitor anode compacts without binder due to the dust generation, abrasive wear, and inadequate pressed compact strength associated with binderless pressing. The problem of adequate removal of binders, necessary for the efficient pressing of capacitor anodes under production conditions, from high surface area anode compacts has been solved in an ingenious manner by Tripp et al. U.S. Pat. No. 5,470,525 describes a method of water leaching of water soluble binders from tantalum anode compacts. This water leaching method of binder removal was extended to water-insoluble acidic binders, such as stearic acid, with the method of using alkali metal hydroxide leach solutions described in PCT No. WO98/38348. This last method suffers from the disadvantage of corrosive attack of the anode bodies if hot and or even mildly concentrated (5%) hydroxide solutions are employed (binder removal is more efficient with hot, concentrated solutions.) In co-pending application, Ser. No. 09/419,893, the problem of anode compact attack by the leaching solution was addressed by the substitution of one or more alkanolamines for the alkali metal hydroxide.
Unfortunately, leaching of capacitor compacts results in partial or total disintegration of the anode bodies when these are pressed from unagglomerated valve material powders. The problem can be overcome through employment of the binder (dimethyl sulfone) and vacuum distillation binder removal method described in our co-pending application Ser. No. 09/397,032.
Although the binder (dimethyl sulfone) and methods of application in the copending application were found to yield minimal residual post-sintering carbon levels with valve metal powders with both water leaching and vacuum distillation binder removal methods, these methods are not ideal from the standpoint of process cost and throughput. Water leaching requires not only a relatively large volume of high purity water, but also a post-leaching drying step to remove the residual water prior to sintering to avoid excessive out gassing during the sintering operation. Vacuum distillation removal of dimethyl sulfone from pressed capacitor anode compacts, while efficient and thorough, requires a closed system capable of withstanding both heat and vacuum, as well a vacuum pump, etc.
The invention is directed to a method of removing a binder, in particular dimethyl sulfone, from pressed compacts, such as anode bodies, comprising heating, preferably to about 100xc2x0 C. and about 350xc2x0 C., the pressed compacts at about atmospheric pressure and circulating or passing a sweep gas over the pressed compacts for a time sufficient to evaporate the binder from the pressed compacts and remove the evaporated binder; wherein the sweep gas is inert to the pressed compacts.
The invention is further directed to the method of removing a binder using nitrogen as the sweep gas.
The invention is further directed to the method of removing a binder using argon as the sweep gas.
The invention is further directed to the method of removing a binder using air as the sweep gas.
The invention is further directed to the binder-free compacts produced using the process of removing a binder described above.
It was discovered that the binder, dimethyl sulfone, can be removed from pressed compacts, such as anode bodies, at atmospheric pressure through the use of heat and an inert cover or xe2x80x9csweepxe2x80x9d gas to both prevent ignition of the anodes and to remove the evaporating binder from the vicinity of the pressed compacts.
Pressed compacts are typically produced from valve metal powders. Suitable valve metals include, but are not limited to, tantalum, niobium, and titanium. Typically, a binder is mixed with the powder prior to pressing and then the powder is pressed to form a compact such as a pellet.
The pressed binder-containing compacts are placed in an oven having inert gas blanketing ability. Preferably, the pressed compacts are placed on an open-frame rack to allow circulation of the atmosphere around the pressed compacts. An inert gas is injected into the oven to provide an inert cover. This inert gas is known as a xe2x80x9csweepxe2x80x9d gas. Depending upon whether the oven is a lab oven or a production oven, the sweep gas is either circulated or passed over the pressed compacts in a single pass. In either case, the sweep gas provides an effective inert cover.
By atmospheric pressure, it is meant pressure values from about local atmospheric pressure to about 125% atmospheric pressure.
The pressed compacts are heated to about 100xc2x0 C. to about 350xc2x0 C., typically to about 125xc2x0 C. to about 325xc2x0 C., more typically about 150xc2x0 C. to about 300xc2x0 C., preferably about 250xc2x0 C. The amount of time needed to remove the binder varies depending on several factors. For example, if the load of pressed compacts is relatively large and/or the temperature employed is relatively low, e.g. 150xc2x0 C., then the time required to remove the dimethyl sulfone binder will be longer than if the pressed compacts are relatively small, the total amount of material to be processed is relatively small, and/or the maximum temperature is relatively high, e.g. 300xc2x0 C.
The type of inert sweep gas depends upon the maximum process temperature and the nature of the material from which the binder containing compacts have been pressed. For most applications, nitrogen is an effective and preferred inert gas. For very finely divided tantalum, niobium, or titanium metals processed at temperatures above 300xc2x0 C., a true inert or noble gas such as argon, helium, or mixtures thereof is preferred. For lower temperature removal of dimethyl sulfone binder from compacts pressed from oxidation resistant materials, such as tantalum nitride or titanium nitride, the sweep gas may be ordinary air. In this case there is very little reaction with the compacts during the atmospheric pressure binder removal step, particularly at binder removal temperatures of 250xc2x0 C. and below. In general, the sweep gas should be relatively inert toward the material the compacts are made of and at the maximum binder removal temperature employed.
The inert gas is injected at a rate suitable for creating the desired atmosphere around the compacts. Generally the rate of injection is between 1 and 1000 cubic feet per hour per cubic foot of oven volume and, typically is 10 to 100 cubic feet per hour per cubic foot of oven volume.